91REDZ28VERT

Ok so my car was parked for years and I have gone through all the fuel system, ignition and electrical system (see my thread "TPI runs really rough").

So any way I got it running good but a few days ago it ran crappy again. Turned out to be a TPS. I was tuning the voltage from .54 to 3.89 and it was still a bit off so I got it to .74 to 4.11 and its good but it has been smelling like fuel since about a week as well.

As I was doing this the fuel canister was making this erupting sound. I discovered it is also the source of the fuel odor. So I pulled the the lines off and it smelled like fuel but it felt like a air hose. So I pull the gas cap hoping to releive the pressure. I open the cap and a huge burst of hot air blows out with lots of pressure it nearly blew the cap out of my hand and burned me. It took about 30 seconds to evacuate all the pressure and it was HOT! I mean way hotter than the exhaust at idle it burned me.

So is this normal? Where should I start? I assume its not safe to drive? The fuel odor is becoming worse, it gets into the ac intake. I appreciate any help thanks!

91REDZ28VERT

Also the fuel odor does not smell like fresh fuel. It smells like old gas that has turned.

Stauf

iTrader: (2)

Therr is a relief valve mounted under the driver rear near the tank. It is possible that yours is bad, allowing pressure to build.
Thare are multiple threads about this same issue. Hundreds of replies and suggestion. And from what I can see, no definitive solution.

Bob88GTA

iTrader: (2)

ANOTHER "Help! Extreme Heat and Pressure in fuel tank"

The search button is broke

91REDZ28VERT

I searched and from what I found there were no specific fixes and some threds were over a few years old. Just posted to see if anyone had any update.

Thanks

Bob88GTA

iTrader: (2)

I wasn't trying to be an azz or anything, it's been one of those topics that we can't seem to get a definitive cure/fix to. Like a bad disease....you carry it around like luggage!

Someday though.

91REDZ28VERT

I understand, there are literally a hundred threads about this with slight variations.

I am pricing the evap system now and will probably just replace it entirley.

These car always have something going on, my tach stopped working this morning.

gormant1

So I have a new theory on this problem that I haven't yet seen anyone put out there. My theory is completely unproven, but the response I got from my car seems promising. Let me know what your thoughts are.

So as some of you know last weekend I installed a new sending unit and fuel pump (both AC Delco parts) in my 91 Formula. Now let me preface my theory by stating that for years I had been dealing with this problem of extreme pressure building in the fuel tank seemingly boiling the fuel (maybe not, but it sounded like it). So after I finished the job I was sitting down in my shop drinking a beer and I started examining the old sending unit. As we all know there are 4 lines that go into the fuel tank sending unit (fuel line, fuel return line, pressure release valve, and the line that goes to the evap canister up front). What I noticed was that the line that goes to the evap canister ends with a (at least what appears to be) small valve on the bottom of the sending unit (which would be inside the tank). I have attached a couple pictures so you can see exactly what I'm referring to. So it got me thinking that perhaps as these things get old, the valves get stuck or maybe lose the ability to open completely, resulting in excessive pressure building in the tank and causing this problem that so many of us have experienced.

Now I have driven my car a few times this week in the blazing Charleston heat and I have had no issues with pressure building in the tank.

So what do you guys think? Does anyone have any concrete information on the little valve that I am referring to? I don't really know for sure, but I wanted to throw this idea out there.

SbFormula

It make sense! Mine builds up quite a bit. It has been since I've bought it in 2001. Always the same constant pressure but nothing overkill. I know for a fact that the EVAP is working. Maybe it's a design flaw and the EVAP doesn't allow enough pressure to be released.

Now in this case, if it has been parked for years, your gas is most likely "rotten" and degraded beyond normal. The pressure build up from years of sitting is probably beyond the capability of the EVAP system. The system was design to eliminate the fuel vapor on a regular basis during regular operation. I would flush that gas and put new one.

Best of luck!

gormant1

Well if it is indeed a valve that opens and closes based on the pressure situation in the tank, and the operational properties of the valve degrade over time like I've theorized, then it would prevent the EVAP system from properly functioning, even if every other component in the system is functioning properly.

And yes the car did sit up for some years, but the fuel is not an issue, as the tank was completely drained and cleaned when the new pump and sending unit was installed.

SbFormula

You're right! But I was referring to 91REDZ28VERT's problem. His car has been parked for years. Sorry for confusion as I quoted your post.

RBob

iTrader: (1)

gormant1, that is a rollover valve, used to prevent fuel from exiting the tank via the CCP purge system. Try blowing air through the CCP vent line, it may be plugged. There is also a restriction is that line in the sending unit.

The CCP system has the lowest pressure relief for the fuel tank. Less then 1 psi.

If the tank pressure goes over that then the vent valve above the rear axle will vent. It should never have to, but it will if required.

The main purpose of the vent valve above the axle is to let air into the tank as it empty's of fuel. This valve is also referred to as the 'mushroom valve' and the 'honk valve.'

When R&R'ing the tank/fuel-pump, it is easy to switch up the CCP vent line and the return line at the tank. Doing so fills the CCP canister with fuel, which at times can pour out of the throttle body. Depends whether the CCP purge solenoid is NO or not.

Note that this information is only for the '90 - '92 f-body's. The systems do vary over the years along with whether the gas cap is vented.

RBob.

91REDZ28VERT

Well I can tell you a $200 canister did not help. It must be something near or in the fuel tank. So I ordered the other under hood components ( just to make sure) fuel pump and sending unit. As I did before in my other post, I will install one part at a time until its gone. I think the sending unit is part of it.

I am building custom ramps to lift the car 14" so I can work on it so it will be a while but I will keep it posted. I may just buy a new tank as well. When all this is done I will have enough parts to build another 91 z28 😀

So far its not been cheap but defenitley worth it.

91REDZ28VERT

Also, I noticed as the fuel level drops the smell gets worse. I fill up drive it for a few days and its fine, light odor but barley detectable. After a half tank with in 15 min I cant use the ac because it smells so bad

Camarothatcould

iTrader: (3)

Just took my car 1300 miles around lake michigan. Halfway through cruising through towns the car would die if in gear. I stopped and let the pressure out of the tank and same exact thing, it nearly blew the cap out of my hand and it purged for probably 20 seconds. Did the rest of the drive with no cap and not a single issue. Only raw fuel smell. Now back home with a new cap the car when hot is still dieing out, going really low idle and if you steer hard or brake hard it will die...

Also trying to figure this out

Camarothatcould

iTrader: (3)

Have any of you ran DTCs? My light was flashing a week or so before all of this was happening

Also right before the trip I bypassed coolant flow to TB

MADMAX350TPI

There are a few things at work here- first, newer fuel formulations have rendered these systems ineffective, however a clean healthy system seems to work well enough- dirty systems with inoperative valves and leaking vacuum lines don't have a chance.

The first issue I would address is the vent line, from the canister all the way to the rollover valve, located under the top of the sender assembly. Replacing the tank assembly and cleaning out the line usually is the best way to maximize the system and make this problem go away. All of the other parts of the system should be replaced, vac lines included, as most of these items are original parts on our cars.

I wonder if swapping out the vent line with a larger diameter tube would help- that thing is tiny, and gets gunky in there...

91REDZ28VERT

Hey guys,

I ordered the ACdelco fuel pump and sending unit, I pulled a good tank out of another Z28 and had it cleaned. I ordered an adjustable fuel pressure regulator and diaphragm. I bought enough vacuum hose to replace all the old lines. Also ordered the egr........ As soon as spring rolls by, I'll start putting all this stuff in. I will also replaced the evap can and solenoid.

91REDZ28VERT

I also bought both Volume 1 and Volume 2 factory service manuals....

ULTM8Z

Have you checked that vent valve (located near the charcoal canister) that goes inline between the canister and the fuel tank on the main vent line?

I actually took mine off a while ago (never could figure out the purpose of it since the charcoal canister is electronically controlled by the ECM.

I used to get a slight build up of pressure (nothing like what you're describing). But after removing that vent valve, I get no pressure build up whatsoever, not matter what the outside air temp is or how long the car has been running.

If your canister is solenoid controlled, make sure the ECM is sending out the 12V to operate the solenoid. Otherwise you'll get no vacuum applied to the tank to vent it.

fred88

Your fuel pump is under extreme pressure because of the filter canister valve or valves not working properly.what I mean is the fuel pump is running harder to maintain the correct fuel pressure.in turn the pump runs hot then even hotter as the tank pressure climbs .check all hoses to and from evap system including the egr lines.put a vacuum gauge on the small hose going to the filter canister and the egr small hose and run the engine .if the vacuum pressure is low when you have a break in the line causing the pressure in the tank to build.hard to believe but true....

FormulasOnly

I had the same issue. I found a kink in the vacuum bypass line (Tank Side Line) that was stopping the inline valve from opening. But like ULTM8Z above said... I'm not sure why GM wanted to double up on the valving there since its electronically controlled

RBob

iTrader: (1)

The solenoid controls the purging of the charcoal canister into the engine. The canister is an open system, as the engine pulls fumes from it air is allowed into the canister. The older canisters have a filter on the bottom where the air is drawn in.

The newer systems have a solid bottom with an air port on the top to allow air in.

At no time does the engine apply vacuum to the purge line going to the tank. It is tank pressure that purges fumes into the canister.

RBob.

FormulasOnly

Its not a "ported" vacuum line but the tank pressure control valve that opens is vacuum operated, once the solenoid opens.

RBob

iTrader: (1)

The tank pressure control valve is not vacuum operated. It is operated by the fuel tank pressure.

Note the canister in your posted image, it has a vent on it. It is open to the air, the canister never has a vacuum in it or pressure within it. The engine never places a vacuum on the fuel tank. The fuel tank pressure is what opens the tank pressure control valve.

Note that this information is only for the '90 - '92 f-body's. As the systems do vary over the years.

RBob.